It is generally practiced to provide the surface of a pigment to be exposed to high temperatures with a heat-resistant coating. For instance, cadmium pigments discolor and lose their brilliant tint when they are exposed to a temperature as high as 700.degree. C. because cadmium sulfide, their main component, is converted to cadmium sulfate or cadmium oxide. Therefore, cadmium pigments cannot be used for products such as chinaware, tiles, etc., which are fired at a high temperature. In order to improve heat resistance of cadmium pigments, they are coated with silicic acid or a water-insoluble silicate salt.
As methods for surface treatment of pigments to improve their heat resistance, the following are known: (a) Forming a coating by adding a silicic acid salt to the medium in which a pigment is formed as a precipitate and thus producing dispersed fine particles of silica, which settle onto the surface of the precipitate to form a coating; (b) Forming a coating by adding a water-soluble silicate to an aqueous dispersion of a calcined pigment, further adding an acid or a water-soluble salt of an alkaline earth metal, zinc, aluminum, zirconium, etc., and forming silicic acid or a water-insoluble silicate to allow them to settle on the surface of the pigment; etc.
As described above, it was well known that heat resistance of pigments is enhanced by coating the surface thereof with silicic acid or a water-insoluble silicate. However, it is difficult to coat the surface of primary particles uniformly and densely by known methods. The above-mentioned process (a) does not give quite satisfactory heat-resistant pigments. In the above-mentioned precipitation method (b), a coating of silicic acid or water-insoluble silicate is formed on the surface of a pigment by precipitation. Precipitation of silicic acid or a silicate is influenced by many parameters such as temperature, pH, etc., and adjustment of these parameters are required and it is difficult to form uniform and dense coating. The coated cadmium sulfide pigments produced by the conventional precipitation methods suffer conversion of cadmium sulfide to sulfate or oxide when fired at 1150.degree. C. for pigmenting. That is, that the pigments have no sufficient heat resistance and cannot be used as a pigment for the intermediate temperature glaze which acquires chemical stability by firing at not lower than 1150.degree. C.
When heat-resistant coating is formed on the surface of a pigment by a precipitation method, the reaction is conducted at an elevated temperature since the surface activity of the pigment is low at temperatures not higher than room temperature. When the reaction is conducted at an elevated temperature, no dense coating is formed. The reason is surmised to be that rapid hydrolysis causes formed SiO.sub.2 to attach itself to the already formed SiO.sub.2 molecules rather than the fresh surface of the pigment, and thus the pigment is not coated uniformly and densely.
We unexpectedly found that uniform and dense coating is formed when a hydrophilic pigment is immersed in a mixture of a metal alkoxide such as an alkyl silicate (silicon tetralkoxide) and a water-miscible or semi-water-miscible solvent, uniform and dense coating is formed; and if this procedure is repeated to form a secondary coating over the primarily formed coating, sounder coating is formed and heat resistance and chemical stability are further improved. The formation of the secondary coating can be performed using a metal salt which forms a water-insoluble hydroxide or oxide or a salt pair which form a water-insoluble salt by a double decomposition reaction.